fix(studio): harden organize-imports against long-running queries and hangs

The original GET route used a correlated NOT EXISTS / FIND_IN_SET
subquery over the entire catalog_items table for every recent import
audit entry, causing server timeouts when the audit log or catalog
grew large.  The per-item host-page validation inside the create
action also issued one SELECT + one UPDATE per moved offer.

Changes:
- GET /api/admin/import/organize: replace the correlated subquery
  with a bounded candidate list and a JS-side placed-set check, then
  resolve all needed base items in a single indexed SELECT.  This
  bounds the query cost regardless of catalog or audit log size.
- organizeImportFurni action: validate mover ids in one SELECT, then
  batch every move per group into a single UPDATE with a CASE
  expression instead of one UPDATE per item.
- OrganizeImportsDialog: add a 45-second abort timeout on the fetch
  and a distinct load-error state so the UI never silently hangs.
- Add 'loadError' translation key (en + nl).

src/messages/nl.json

@@ -1972,6 +1972,7 @@
 						"itemsBadge": "{count} items",
 						"loading": "Geïmporteerde meubels laden…",
 						"empty": "Geen geïmporteerde meubels gevonden. Importeer eerst meubels.",
+						"loadError": "Kon imports niet laden. Probeer het opnieuw.",
 						"refresh": "Vernieuwen",
 						"desc": "Geïmporteerde meubels worden gegroepeerd in voorgestelde pagina's. Bekijk elke groep en maak alleen de pagina's aan die je goedkeurt.",
 						"selectAll": "Alles selecteren",